Disk drive with redundant recording

ABSTRACT

A magnetic disk drive includes redundant data written at a plurality of out of phase angular locations to reduce the latency and enhance performance during a read operation. The loss of recording capacity is reduced by increasing the data density to achieve the same soft error rate standard required for single recording. Dual recording also allows different recording codes to be used at the duplicated locations to thereby have the more highly stressed code words occur at different locations in the data to further reduce the possibility of an error. The redundant recording can be used in one portion of the media and normal recording used in another media portion to enable selection of the recording technique in accordance with the type of data being stored. The size of the normal and redundant recording portions can be controlled by the format operation and the user of the disk drive can intervene to designate the size of the redundant and normal media recording portions effected during the format operation.

BACKGROUND OF THE INVENTION

The present invention relates to disk drive data storage devices. Moreparticularly, the present invention is directed to a disk drive withreduced latency to optimize performance and data throughput.

As the speed and performance of data processing systems is increased itis necessary to enhance the performance of disk data storage devicesthat include mechanical as well as electronic functions. Criticalaspects are the access time as the transducer moves from track to trackand the latency as the rotating media revolves to the position of thedata addressed on the track. The latency problem can be resolved byhigher rotational velocities of the media, but this is limited byconstraints such as recording channel bandwidth, power consumption andspindle bearing life. These factors are particularly critical in smallerform factor devices wherein smaller media must rotate at a highervelocity to obtain the same linear velocities obtained using a largerdiameter media and in portable devices where the size of the portablepower supply is a criteria.

In various prior art devices the latency problem is addressed byredundant recording on the same or different tracks, the use of multipletransducer heads or most usually, a combination of multiple heads andredundant recording. U.S. Pat. No. 2,680,239 teaches the use of multipletransducer heads which address the same track. A switching deviceactivates the head nearest the data of interest. In U.S. Pat. No.3,103,650 multiple heads are used and in one embodiment four heads areused in conjunction with triplicate recording to read data stored on atrack in one-twelfth of a revolution. U.S. Pat. No. 3,729,725 includestwo transducer heads that write in separate regions (different tracks)with the data written by the second transducer head delayed by 180degrees of rotation to assure that the beginning of the redundant datais displaced 180 degrees. A dual actuator system is used in U.S. Pat.No. 4,270,154 wherein the actuators are positioned 180 degrees apartaround the disk periphery so that any read or write operation is done bythe actuator that can first commence the operation.

SUMMARY OF THE INVENTION

In the device of the present invention only a single transducer isutilized to address each track. The data is redundantly recorded on eachtrack with the duplicate recordings angularly separated by 360 degreesdivided by the number of redundant recordings. The most practicalembodiment is the dual recording of data written at locations 180degrees apart. This reduces the latency associated with a read operationby half, but actually increases the duration of the write operationbecause of the duplicate recording.

Although it would appear that the recording capacity of the device wouldbe reduced by half as a result of the dual recording of data, such isnot the result. Since the data is recorded twice, a higher soft errorrate (SER) can be accepted in each of the redundant fields whileachieving an overall soft error rate that is acceptable. For example,dual recording enables a recording density 36% greater while maintainingthe SER of the single recording technique. Thus the reduction of storagecapacity is 32% rather than 50% with respect to this aspect alone. Inaddition, the duplicate recording enables other storage capacityimprovements. For example, with most devices utilizing run lengthlimited (RLL) codes for recording data, different RLL codes can be usedfor initial recording and duplicate recording of the data. By selectingRLL codes with different code words that are most highly stressed, thelikelihood of errors in the same portion of the duplicated data isreduced which also permits increased recording density with the sameSER.

In addition, the user can choose to operate the drive in single mode atfull capacity, redundant mode at reduced capacity, or in a combinationmode with some single and some redundant modes within the same drive.This definition is determined when the user prepares the drive duringthe formatting process and can be changed by reformatting the drive toanother configuration.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic showing of a disk drive media surface wherein atrack has been redundantly recorded 180 degrees out of phase. FIG. 2shows a single sector of a data track. FIG. 3 is a showing similar toFIG. 1 wherein a mixed format of redundant and normal recording is used.FIG. 4 is a block diagram of a disk drive control system including theredundant recording technique of the present invention.

DETAILED DESCRIPTION

The disk drive industry trend is for specific user applications torequire ever increasing performance, which is satisfied by increasingspindle rotational velocities and reducing access times. The need forimproved access speed and data rate performance places demands onspecific form factor constraints such as the spindle and actuator mass,power consumption and bearing life. At a given form factor, the cost ofimproving these performance numbers by a significant percentage issubstantial. This invention is directed to solving the problem ofreducing latency in disk storage devices without increasing therotational speed of the device and with minimal impact on the storagecapacity and error rate performance.

Although the data could be replicated any number of times at angularlydisplaced locations, the invention is illustrated by writing data twiceon the same track at locations 180 degrees apart. Using this format, thelatency or average time for a head on the track to reach desired data onthat track is reduced by exactly 50%. It is recognized that this formatonly enhances performance for the read operation, while the writeoperation requiring that the data be written twice will actually resultin a longer delay. During the read operation, with track split in halvesof redundant data, the mean latency for the first occurrence of thedesired data is 0.25 of the time for one revolution. Ordinary latencywithout the use of redundant data is 0.5 the time for one revolution ofthe spindle. For the write operation with two redundant track halves,the average latency is 0.75 of the time for one revolution. Thus theperformance depends on the read to write ratio. In an environment wherefour reads per write occur, the use of two redundant zones would reducelatency by 30%. While a one to one ratio would result in no saving, anine to one ratio would achieve a 40% saving.

A limitation to the use of redundant data is the time delay caused by ahead switch that occurs more often since it is necessary to switch headsevery half revolution rather than following a full revolution. Thislimitation results in a reduced saving as the size of the data blockbeing transferred increases.

To the first order, the above concept involves a 50% loss in capacityusing the conventional mindset about a design point. However, given thatdata is written twice on the disk, it is possible to use a smaller trackpitch, higher linear density or both to offset this loss of capacity.This can reduce the loss of storage capacity at the same error rate.

Assume that the magnetic design is optimized in the classical sense witha track density and a linear density for a given acceptable error rateperformance. This represents the capacity that can be achieved using theconventional approach and is therefore the benchmark for the subsequentcomparison.

In simple terms, the capacity loss occasioned by double recording of thedata can be completely recovered if it is possible to double either thetrack density or the linear density. A more likely option is a suitablecombination of increases in both densities. An accurate estimate of whatcan be achieved in terms of capacity at the same error rate performancewould require the convolution of the bathtub curve (soft error rateversus head off-track curve) associated with a doubling of data on atrack with the track misregistration (TMR) distribution. From a testviewpoint, the data would have to be written twice on a track and asuitable algorithm developed to handle the second attempt at readingdata after an error is encountered. With these features in place, theoptimization of linear density and track pitch would proceed in the samemanner as in the classical approach. Errors would be registered only ifthey occur on both attempts to read the same data in one revolution.Curves would be generated based on higher linear and track densitydesigns until the benchmark curve is approximated. Under this conditionthe SER will closely match that of the benchmark.

In the absence of the above test data, it is possible to sense theability to achieve higher areal densities to offset the duplication ofdata by the following argument. Suppose that for a given linear densitythe SER achieved is some number, say 10⁻¹⁰. If the data on the track isrepeated once while maintaining the linear density fixed, then the SERtheoretically possible is the probability of having the error twice.From probability theory, this is the product of the original error ratewith itself or 10⁻²⁰ . Hence lost capacity due to duplication can berecouped by increasing the linear density until the SER degrades back to10⁻¹⁰.

This concept is readily accomplished using existing hardware withappropriate modifications to the microcode. For most applications thestring of information to be recorded is typically small in length so thedouble write operation will be completed with essentially no overhead.For longer strings, a write cache would buffer the data until the drivecan resume doubling the data.

Even if the recovery of capacity lost as a result of duplication is not100%, there are many applications where throughput far outweighscapacity. In that case this invention offers a strategic solution. Froma power and spindle speed viewpoint, it is becoming more and moredifficult to make significant advances that translate into improved datathroughput performance. In some applications, the power required forhigh rotational velocities is prohibitive. Moreover as spindle bearingsare being designed to run faster and fit into smaller form factors,bearing life is found to drop rapidly with rotational speed. Hence, thisinvention provides a relatively simple solution to lowering latency whenhardware and power considerations are reaching apparent technicalbarriers.

Also, in many specific applications the standard requirements forextremely low SER are not necessary. In particular, applications such asmultimedia, animated graphics, etc, where the output is pictures on ascreen, the SER could be orders of magnitude higher since the eye isforgiving at that scale. Hence it is possible to be much more aggressiveregarding linear density (and optimize the design point write to channelcapability and RPM) so latency is improved without impacting capacity.

FIG. 1 is a schematic representation of a recording media disk surfacethat has been written using 100% redundancy including sectors A throughL recorded on one half of a track and duplicated by sectors Ar throughLr written on the other half of the track with the redundant sectorsseparated by a 180 degree phase shift. As shown in FIG. 2, each sectorhas a header portion which includes identification of the media surface,track number, sector number, format and coding information and a dataportion in which the user data to be stored is written.

FIG. 3 is a schematic showing of a disk data surface incorporating amixed format including tracks with the 100% redundant format whereindata stored in sectors A through L is duplicated in sectors Ar throughLr on the other half of the track. Other tracks include a normal formatwhere sectors A through X occupy the entire track. Thus the drive may beused totally in the normal mode or totally in the redundant mode or oneportion or zone can be formatted for redundant recording while anotherzone is formatted for normal recording as in FIG. 3. The trade offoption between the high capacity of normal recording and the highperformance of redundant recording may be chosen by the user. Thisability provides a drive which can be customized to the user's needs andcan be changed to accommodate changing requirements.

The block diagram of FIG. 4 includes a microprocessor controller 14which exercises overall control of the disk drive including spindlemotor control logic 17 to regulate the rotational speed of disk stack 13and actuator servo control logic 18 which positions the transducer inalignment with a selected media track and maintains track alignment. Theangular position is sensed from the signal from the servo head in driveswhere a dedicated servo surface is used or from the individual surfaceservo information within the sector header in drives which use anembedded or sector servo system. Data to be written is received atinterface 9 from a host system on data bus 10 and processed by channelcontrol logic 11 which senses the type of data and sets the parametersas to single or dual recording and the recording density to optimize theoverall recording density and performance in accordance with the errortolerance of the type of data being recorded. For dual recording, therecording code (encode/decode 1 or 2) is selected in accordance with theangular position of the disk stack and encoded serial data is receivedby the channel 15 for transmission to the transducer head 16 forrecording on a media track on disk stack 13. If the single recordingmode is to be used, the recording code is selected. With theavailability of dual encoding methods, the default code for a singlerecording format can be selected at the time of manufacture to be thecode which is most effective for each transducer-media surfacecombination. Thus it is possible to further accommodate media and headvariables such as heads which are narrower or wider within themanufacturing tolerance.

When reading data stored on the media of disk stack 13, the serial datareceived on channel is decoded using the code indicated by the angularposition, data type and selected default code in the environment ofsingle recording.

In direct access storage devices such as rigid magnetic disk drives, ithas been found that a 6% increase in the linear recording densityresults in a corresponding order of magnitude (factor of 10) increase inthe soft error rate (SER). Thus if the soft error rate standard is 10⁻¹²and 100% redundancy is used, the error rate in each of the recordedareas can be 10⁻⁶ to maintain an overall SER of 10⁻¹². The error rate of10⁻⁶ is six orders of magnitude less than the 10⁻¹² standard allowingfor a six times 6% or 36% increase in the linear recording density whilemaintaining the same overall error rate.

For a typical disk drive rotating at 4500 RPM (6.66 millisecondslatency) with a capacity of 1.2 GB assuming an on track error rate of10⁻¹², 100% redundancy would result in a disk drive with 816 MB capacitywith a latency of 3.33 milliseconds while maintaining the same errorrate standard. Thus the user must sacrifice some capacity, but not 50%,to cut the latency time in half. If zone bit recording (ZBR) was used inthis case with a capacity gain of 25%, the storage capacity of such adrive would be over 1 GB. The alternative would be to spin the spindleat 9000 RPM increasing the spindle power consumption by a factor of 4and reducing the bearing life.

What is claimed is:
 1. A data recording device wherein data is recordedin parallel tracks that repetitively pass a transducing locationcomprisinga recording media; transducing means positionable to writedata on and read data from said media, said transducing means accessingdata on one and only one of said parallel tracks at a time; means forrecording data on said device a multiplicity of times at out of phaselocations, said means for recording data at each of the multiplicity oflocations recording said data at a density that exceeds the density atwhich the data can be read at an acceptable level of errors, whereby ifan error occurs, the data can be read at the alternate recordinglocation and the error rate after reading at each of the multiplicity ofdata locations is within said acceptable level of errors; means foraccessing data recorded on said device a multiplicity of times byselectively accessing one of said multiplicity of out of phaselocations, said one of said multiplicity of out of phase locations beingselected based on latency time required to access data stored at each ofsaid multiplicity of out of phase locations; and means for effectingeither single recording or multiple recording on a given track on saidmedia.
 2. The data recording device of claim 1 wherein said recordingdevice is a magnetic disk drive and data is duplicated on the same trackon said media.
 3. The data recording device of claim 2 wherein dataduplicated on the same track 180 degrees out of phase from the initialrecording location.
 4. The data recording device of claim 3 wherein theduplicated data is initially written at a first location using a firstencoding system and is duplicated by being written using a secondencoding system, said second encoding system being different from saidfirst encoding system.
 5. The data recording device of claim 4 whereinsaid first and second encoding systems are different forms of run lengthlimited codes.
 6. The data recording device of claim 1 furthercomprising control means, said control means being operable to formatsaid recording device to designate a first portion of said recordingmedia for normal data recording and a second portion of said recordingmedia for redundant recording of data at a multiplicity of out of phaselocations.
 7. The data recording device of claim 6 wherein a usercommand is operable to define the size of said first and second portionsof said recording media effected by a format or reformat operation.
 8. Adata recording device wherein data is recorded in parallel tracks thatrepetitively pass a transducing location comprisinga recording media;transducing means positionable to write data on and read data from saidmedia; means for recording data on tracks of said media a plurality oftimes at substantially uniform angular displacements; and means foreffecting either single recording or multiple recording on a given trackon said media.
 9. The data recording device of claim 8 wherein saidmedia is a rigid disk magnetic media andsaid means for recording dataincludes means for twice recording data on a single track at first andsecond track locations 180 degrees out of phase from one another. 10.The data recording device of claim 9 wherein said means for recordingdata causes data to be recorded at said first track location using afirst run length limited code and at said second track location using asecond run length limited code.
 11. The data recording device of claim10 wherein said data written at said first and second track locations isrecorded at a linear density that exceeds the density at which data canbe read with an acceptable level of errors,whereby if an error occurs,the data can be read at the alternate recording location and the errorrate after reading at each of the locations is within the acceptablelevel of errors.
 12. The data recording device of claim 8 furthercomprising a control means including a format function, said formatfunction operable to designate a first portion of said recording mediafor normal recording and a second portion of said recording media forredundant recording wherein data is recorded at a multiplicity of out ofphase locations.
 13. The data recording device of claim 12 wherein saidcontrol means is responsive to a user command to designate the size ofsaid first and second portions effected by a format operation.
 14. Acontrol apparatus for a data recording device wherein data is recordedin parallel tracks on a recording media that repetitively pass atransducing location, said data recording device having a single datatransducer capable of accessing data on one and only one of saidparallel tracks at a time, said control apparatus comprising:means forpositioning said data transducer to write data on and read data fromsaid media; means for recording data on said media a multiplicity oftimes at out of phase locations, said means for recording data at eachof the multiplicity of locations recording said data at a density thatexceeds the density at which the data can be read at an acceptable levelof errors, whereby if an error occurs, the data can be read at thealternate recording location and the error rate after reading at each ofthe multiplicity of data locations is within said acceptable level oferrors; means for accessing data recorded on said device a multiplicityof times by selectively accessing one of said multiplicity of out ofphase locations, said one of said multiplicity of out of phase locationsbeing selected based on latency time required to access data stored ateach of said multiplicity of out of phase locations: and means foreffecting either single recording or multiple recording on a given trackon said media.
 15. The control apparatus for a data recording device ofclaim 14 wherein said data recording device is a magnetic disk drive,and said means for recording data a multiplicity of times records data amultiplicity of times on the same track on said media.
 16. The controlapparatus for a data recording device of claim 15 wherein said means forrecording data a multiplicity of times duplicates data on the same track180 degrees out of phase from the initial recording location.
 17. Thecontrol apparatus for a data recording device of claim 14 wherein saidmeans for recording data a multiplicity of times records data at a firstlocation using a first encoding system and records said data at a secondlocation using a second encoding system, said second encoding systembeing different from said first encoding system.
 18. The controlapparatus for a data recording device of claim 14 further comprisingcontrol means, said control means being operable to format saidrecording device to designate a first portion of said recording mediafor normal data recording and a second portion of said recording mediafor redundant recording of data at a multiplicity of out of phaselocations.
 19. A control apparatus for a data recording device whereindata is recorded in parallel tracks on a recording media thatrepetitively pass a transducing location, said control apparatuscomprising:means for positioning a data transducer to write data on andread data from said media; means for recording data on tracks of saidmedia a plurality of times at substantially uniform angulardisplacements; and means for effecting either single recording ormultiple recording on a given track on said media.
 20. The controlapparatus for a data recording device of claim 19 wherein said media isa rigid disk magnetic media andsaid means for recording data includesmeans for twice recording data on a single track at first and secondtrack locations 180 degrees out of phase from one another.
 21. Thecontrol apparatus for a data recording device of claim 20 wherein saidmeans for recording data causes data to be recorded at said first tracklocation using a first encoding method and at said second track locationusing a second encoding method, said second encoding method beingdifferent from said first encoding method.
 22. The control apparatus fora data recording device of claim 20 wherein said data written at saidfirst and second track locations is recorded at a linear density thatexceeds the density at which data can be read with an acceptable levelof errors,whereby if an error occurs, the data can be read at thealternate recording location and the error rate after reading at each ofthe locations is within the acceptable level of errors.
 23. The controlapparatus for a data recording device of claim 19 further comprisingformatting control means including a format function, said formatfunction operable to designate a first portion of said recording mediafor normal recording and a second portion of said recording media forredundant recording wherein data is recorded at a multiplicity of out ofphase locations.